The invention relates to a circuit arrangement for driving a semiconductor device connected in series with a load across a power supply, the arrangement comprising means for applying appropriate signals to a control electrode of the semiconductor device in response to a control signal defining the desired state of conduction of the semiconductor device, means for detecting if the power supply voltage exceeds a predetermined threshold value and means for causing the semiconductor device to conduct by connecting the control electrode to a first terminal of the power supply when the threshold detecting means detect that the supply voltage exceeds the threshold value.
Such circuit arrangements are required in many applications for driving output transistors, thyristors, triacs and the like in electric power circuits. In a particular example, the semiconductor device and its associated circuit arrangement may act as an electronic switch for controlling a lamp or an electric motor.
In some applications, particularly where inductive elements form part of the system, such as in a motor vehicle, the voltage supply may be subject to high-voltage transients, or `spikes` which could damage a semiconductor switch more easily than a traditional electro-mechanical switch. Excessive voltage leads to breakdown of junctions within the semiconductor device and possibly to permanent damage.
United Kingdom Patent Specification No. GB1534206 discloses such an arrangement and aims to alleviate this problem by turning on the semiconductor device when the supply voltage exceeds the threshold value regardless of the otherwise desired state.
The known arrangement makes use of the fact that the semiconductor device, a transistor, is more susceptible to damage by the transients when turned off (not conducting) because it then has the full voltage of the supply and transient applied across its main current carrying terminals. During a high voltage transient which causes a breakdown current to flow through the turned-off semiconductor device, this large voltage drop means that a large amount of energy may be dissipated within the semiconductor, causing damage by heating. In contrast, when the semiconductor device is on (conducting) the supply voltage (and therefore any transient superimposed upon the supply voltage) is applied mainly across the load being controlled, which will be constructed so as to withstand the transients as part of its normal operation. By turning the semiconductor device on during periods of excessively high voltage, even when the otherwise desired state of the device is the `off` state, the load is made to drop at least some of the extra voltage. This reduces the heat energy generated within the semiconductor device to avoid damage. To provide effective protection, it is known that the threshold detecting means should act independently of the normal driving circuitry, connecting the control electrode as directly as possible to the first terminal of the supply in the shortest possible time.
A problem arises with the known arrangement where the application dictates that the seiconductor device must turn off quickly in response to a control signal. To achieve a fast turn-off, the means for applying signals to the control electrode may comprise active turn-off means for connecting the control electrode to a second terminal of the power supply when the desired state of conduction is `off` (not conducting). This is effective to discharge quickly the large capacitance which is inevitably associated with the control electrodes of large power devices. For example, power metal-oxide-semiconductor field-effect transistors (MOSFETs) may have a gate capacitance of several tens or even hundreds of picofarads (pF), depending on the size and type of the power device.
However, combining the known directly-acting protection arrangement described above with an active turn-off means causes the problem that the turn-off means may be liable to damage by high currents during the ocurrence of a spike, because the protection arrangement serves to apply the full transient voltage across the turn-off means (for example a conducting pull-down transistor). One solution is to make the turn-off means have a sufficiently high resistance such that the transient current is limited to a safe level. A known circuit adopting this solution is Siemens' BTS412 intelligent power switch integrated circuit, described by J. Tihanyi and M. Glogolja in the Conference Record of the 1986 I.E.E.E. Industrial Applications Society Annual Meeting, at pages 429 to 433. See particularly the resistance RG in FIG. 2 on page 430. However, the high resistance required makes the turn-off arrangement less effective in achieving fast turn-off of the semiconductor device being driven by the arrangement.